Downhole "mud motors" have long been recognized as an economical tool for carrying out a variety of functions within a weilbore, e.g. milling a window in a well casing, drilling a lateral wellbore into a formation, etc. As will be understood in the art, a downhole mud motor is typically a positive displacement, hydraulic rotary motor which is lowered into a wellbore on the lower end of a toolstring and is actuated by pumping a power-fluid (e.g. drilling mud, water, etc.) down the toolstring and through the motor. As the fluid flows through the motor, it causes the motor to rotate at a speed which is proportional to the flow therethrough.
While mud motors offer several known advantages over more conventional rotary drilling methods in many applications, there are situations where their use may be detrimental. For example, downhole mud motors are commonly used with diamond mills (i.e. bits) to mill windows in cased wells and are then used to drill lateral bores or drainholes out into the surrounding formation through the milled window(s). Diamond mills are highly effective for this purpose but, as will be recognized, their use is relatively expensive. Recently, it has been proposed to use mills which have tungsten carbide cutting surfaces. These less-expensive, carbide mills actually perform more efficiently in many milling operations than do the diamond mills.
Unfortunately, however, carbide mills may actually cut too well when used with bent-sub, downhole motor combinations or with bent motors, the latter being a type of known downhole motor whose housing is bent at an angle along its length. The bend in the sub or the motor housing provides the side force needed by the mill to initiate a cut in the casing and also it establishes the direction in which the mill is to cut the window in the casing and which a lateral is to be drilled.
Known toolstrings commonly used in carrying out such milling/drilling operations are typically made-up of the following: a workstring which extends into the well from the surface and a downhole assembly carried on the lower end of the workstring. The downhole assembly, in turn, is basically made-up of at least (a) a bent-sub, downhole motor combination or a bent-housing motor (hereinafter collectively called "bent motor"); (b) a mill/bit connected to the output of the motor; (c) a sensor for detecting the azimuth of the bent motor and generating data representative thereof; and (d) a means for transmitting the azimuth data to the surface. As will be understood in the art, this latter means may comprise either a cable (e.g. E-line) extending to the surface or a measurement-while-drilling {MWD} tool which transmits data through the liquid column in the wellbore.
Where rigid drill pipe is used as the workstring to lower the downhole assembly, the bent motor may be rotated to its desired azimuth/direction as determined by the downhole data by merely rotating the rigid workstring at the surface. However, recently, coiled tubing is used more and more as the workstring in such operations. As will be understood in the art, "coiled tubing" is a continuous length of a relatively small diameter (e.g. 3/4-31/2 inch), thin-walled metal tubing (e.g. steel, etc.) which can be wound or coiled onto a reel or spool and which can be paid out or reeled in without having to make-up or break-out individual stands of pipe. Coiled tubing is well known in the art and is readily available in the field. However, since the tubing is coiled onto a reel, it can not readily be rotated from the surface as will be understood in the art. Accordingly, the downhole assembly must now also include an orienting tool (i.e. "orienter") which can be manipulated from the surface to orient the bent motor to its desired azimuth/direction before the milling/drilling operation can be carried out.
In a window-milling operation, a toolstring, such as described, above is lowered down a well until the mill on the lower end of the bent motor is at the depth at which the window is to be milled in the well casing. A work-fluid (e.g. drilling mud, water, etc.) is circulated down the workstring and through the downhole assembly (e.g. an orienter, MWD tool, and a bent motor). The fluid flows out of the motor, through the mill, and back to the surface through the annulus formed between the well casing and the tool string.
Once the mill is at the desired depth, the work-fluid has to be circulated through the downhole assembly for several minutes before the actual cutting of the window can begin in order to (1) sense the azimuth/direction in which the bent motor and mill are initially pointed; (2) transmit that data indicative of the sensed direction to the surface by means of the MWD tool; and (3) cycle the work-fluid pumps at the surface to cause the orienter to rotate (i.e. ratchet) the bent motor around to the desired direction.
Unfortunately, the work-fluid also flows through and powers the motor during the time it takes to orient the bent motor. This is not usually considered a serious problem where diamond mills are being used because diamond mills are slow to initiate the cut required in milling a window. Accordingly, rotation of a diamond mill during the orienting of the bent motor will cause little, if any, damage in the wellbore.
However, since carbide mills can cut much faster than diamond mills, rotation of the mill (i.e. powering the downhole motor) while orienting the bent motor can cause substantial damage to (a) the cement plug normally present in the wellbore, (b) the liner or casing, and (c) even to the mill, itself, before the mill is properly oriented in the well. To alleviate this problem, it has been proposed to coat the cutting surfaces of carbide mills with a relatively soft, sacrificial metal (e.g. brass, lead, etc.) which is designed to wear away and eventually allow the desired carbide cutting surfaces to become exposed.
In practice, however, it is difficult to place the right amount of this metal in just the right spots to insure that the sacrificial metal will wear consistently. That is, sometimes the sacrificial metal wears away faster than expected or the orientation period takes longer than anticipated thereby resulting in damage due to early exposure of the cutting surfaces of the rotating mill. In other instances, the sacrificial metal never completely wears off of the cutting surfaces thereby interfering with the initiation of the cut required to successfully mill a window in the wellbore.